In designing an engineering system, a practitioner may use a graphical model comprised of connected building blocks to simulate the system behavior and calculate system performance indicators with computers. The model may include building blocks that may be physical in nature, such as mass, springs, electrical resistors, and the like. Such models may embody laws of physics and may be called physical models. Software systems known as lumped-parameter dynamic systems simulators can process a graphical model to create graphs depicting system behavior. Numerical results may be further processed to yield measures of effectiveness (MoE) of a design. Examples of MoEs may include fuel economy, fatigue life, noise level, etc.
MoEs may be related to physical variables such as velocity, temperature, or voltage. Other criteria, such as, for example, cost, weight, or reliability targets, may also be used as MoEs. For reliability calculations, the practitioner may use another type of graphical model, called a fault tree or reliability block diagram (RBD), to calculate the likelihood that a system can deliver its intended functions. RBD may include connected building blocks that may be characterized by failure rates, such as mean time between failures. Reliability models may also differ from lumped-parameter dynamic models in how the blocks are processed to yield the MoEs.
For cost and weight analysis, the practitioner may use spreadsheets or similar software that may provide accounting functions. A practitioner may perform a trade-off analysis involving cost, weight, reliability, and system performance.